Grey egg?

Our three new layers were broody-raised. Dad was a Welsummer and the moms were 2 EEs and another Wellie. We're pretty sure the three on the top of the picture are from those three. The two olive ones we figure are the Wellie x EE hens. The grey one (it's not as blue as it looks in the picture, it really is a grey with a slight bluish tint) we think must be from the "pure" Wellie. Do you think we're right? Both Wellies were from hatchery chicks, so I know we can't be sure they're 100% pure. Has anyone ever had a grey egg? I did some searches but came up empty. If so, you think it's the Wellie/Wellie, or is it more likely one of the olives came from her?